Multilayer, biaxially oriented polyester film, process for its production and its use

ABSTRACT

Three-layer, highly oriented polyester films having an A-B-C layer construction, preferably made of polyethylene terephthalate, which, in addition to a base layer B have a smooth surface layer A which contains no external particles and a rough surface layer C which contains external particles such as antiblocking agents or pigments, preferably SiO 2 , in defined amounts and particle sizes, and the surface layer C is additionally provided with an acrylate-containing layer D feature in particular good producibility and processibility, high gloss of the film surface A and low opacity. The very smooth surface A is suitable in particular for coating with materials, for example SiO x , by means of plasma polymerization of hexamethyldisilazane by the CVD or PECVD method, which provides the film with a high oxygen barrier and is suitable in particular as packaging for light- and air-sensitive foods or other consumable items.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a transparent, preferably three-layer,biaxially oriented polyester film having a smooth surface A which hassubstantially no external particles, and a rough surface C whichcontains antiblocking particles in a certain size. In addition, the filmhas at least one acrylate-containing layer D which is applied to thesurface C as an aqueous dispersion. To attain a very high surfacesmoothness of the surface A, the film is highly oriented. The film issuitable in particular for barrier coatings which are applied by the CVD(chemical vapor deposition) or by the PECVD (plasma enhanced chemicalvapor deposition) method. The invention further relates to a process forproducing the film and to its use.

[0002] Films having a smooth surface A are disclosed by the prior art.

[0003] For instance, films in accordance with examples 4 and 6 of EP-A-0903 222 contain no external pigments in the surface layer A. The filmaccordingly has a smooth surface A. According to experiments carried outthere, the film has good barrier values, in particular aftermetallization or coating with ceramic materials. For barrier coating bythe CVD or the PECVD method, the smoothness of the surface A to becoated is in need of improvement. Also in need of improvement is theprocessing performance of the film, in particular on high-speedmachines. The film according to the aforementioned examples 4 and 6 isnot adequately pigmented on the surface layer C on the opposite side tothe surface layer A. In the case of coating in high-vacuum units, forexample in units which work by the CVD or by the PECVD method, the unitis evacuated before the coating. This also allows air present in thefilm roll to escape, resulting in the individual film plies lying inclose contact. Inadequate pigmentation may result in poor runningperformance of the film in the machine (pulling on the bias, blocking).The winding quality of the coated film is unsuitable for the furtherprocessing of the film (lamination, printing). Such a film also has avery strong tendency to electrostatic charging.

[0004] DE-A-16 94 404 describes a film composed of layers of an orientedcrystallizable thermoplastic film, in which at least one of the outerlayers contains an additive. The additives are customary inert inorganicor organic particles which, in the case of inert particles such as SiO₂are added to the outer layers in a concentration of from 1 to 25% byweight. The particle size is from 2 to 20 μm. These laminates can beused, for example, for decorative purposes metalized with aluminum orfor magnetic tapes. There is no information whatsoever in the documentas to how the topography of such a film can be adjusted for theimprovement of the oxygen barrier.

[0005] DE-A-22 30 970 describes a magnetic recording medium whichconsists of a biaxially oriented polyester film and a thin magneticmetallic layer on the surface A of the polyester film. The film has

[0006] a) a coated surface A which is free of particles and

[0007] i) is at least 4 μm thick or

[0008] ii) is at least 50% of the thickness of the overall film ply; and

[0009] b) a particle-containing second layer having a comparativelyrough surface which contains

[0010] i) at least 1% of individual particles of a certain polymer A and

[0011] ii) at least 1% of individual particles of a certain polymer B.

[0012] There is again no information whatsoever in the document as tohow the topography of such a film for the improvement of an oxygenbarrier can be attained.

[0013] EP-B-0 088 635 describes a coextruded, biaxially orientedpolyester film having at least two layers of which one layer A consistsof thermoplastic resin and one layer B which comprises thermoplasticresin and fine particles. The film has a surface roughness R_(a) of theouter surface of layer A of less than 5 nm and the outer surface oflayer B has either

[0014] a surface having a surface roughness R_(a) of from 5 to 40 nm anda plurality of depressions and a plurality of protrusions which arearranged in a certain arrangement, or

[0015] a surface having protrusions formed on a flat plane and whosesurface is covered with a layer C composed of a lubricant and having asurface roughness R_(a) of from 5 to 40 nm.

[0016] EP-A-0 514 129 describes a transparent multilayer film whichcomprises a primary layer substrate of polymer material which, on atleast one of its surfaces, has a secondary layer of polymer materialwhich has glass beads and silicon dioxide particles in certainconcentrations and in certain size distributions. The secondary layermay be disposed on one or on both sides of the primary layer substrate.The film improves the opacity and the processing properties, but thedocument does not provide any teaching on the improvement of the glossand of the barrier properties of the film. There is also no informationwhatsoever in the document as to how the topography of such a film forthe simultaneous improvements of gloss and oxygen barrier is to beattained.

[0017] EP-A-0 604 057 describes a transparent multilayer film whichcomprises a primary layer substrate of polymer material which issubstantially free of fillers and, on at least one of its surfaces, hasa secondary layer of polymer material which contains silicone resin as afiller in a concentration of from 100 to 1000 ppm and an averageparticle diameter of from 1.5 to 12.5 μm. A disadvantage of the siliconeparticles is that they are comparatively expensive and do not constitutean acceptable solution for the packaging market. In addition, filmswhich are equipped with such pigments have a more marked tendency totelescope on winding. There is likewise no information whatsoever inthis document as to how the topography of such a film for thesimultaneous improvement of gloss and oxygen barrier is to be attained.

[0018] As a consequence of the lacking smoothness of the surface A to becoated, the films described in the prior art are unsuitable for barriercoating by the CVD or by the PECVD method, or are at least in need ofimprovement. Likewise in need of improvement is the processingperformance of these films on these high-vacuum units.

BRIEF DESCRIPTION OF THE INVENTION

[0019] It is an object of the present invention to provide atransparent, biaxially oriented polyester film which is suitable inparticular for barrier coating by the CVD or by the PECVD method. Afterbarrier coating with substances which are used in the CVD or by thePECVD method (for example plasma polymerization of hexamethyldisiloxane,CH₄) the film should have the desired high oxygen barrier.

[0020] In addition, the required good winding quality of the film shouldnot be impaired by the coating procedure; it would be desirable for thefilm to only insignificantly charge electrostatically during the coatingprocedure and in all subsequent processing steps. Moreover, the filmshould feature good producibility and processibility. In summary, theobject was to provide a film having the following combination offeatures:

[0021] high oxygen barrier (meaning low oxygen permeation) of the filmafter barrier coating of the film surface A by the CVD or by the PECVDmethod (utilization of plasma polymerization)

[0022] good preparability, in particular good windability and good rollformation, and also good processibility

[0023] high gloss of the film surface A to be coated (high surfacesmoothness)

[0024] high transparency, i.e. low opacity.

[0025] The film should be at least equivalent in the remainingproperties to the existing packaging films of this type. It should bepossible, for example, to produce them simply and inexpensively, and toprocess them efficiently on the conventional machines.

DETAILED DESCRIPTION OF THE INVENTION

[0026] This object is achieved by a biaxially oriented polyester filmwhich has a base layer B composed of at least 80% by weight of athermoplastic polyester, and has two outer layers A and C, wherein

[0027] a) the outer layer A has high surface smoothness and comprisessubstantially no external particles,

[0028] b) the outer layer C comprises external particles and has, permm² of film surface area, a number of elevations N_(c) which correlateby way of the following equation to their respective heights h

A _(h1) −B _(h1)·log₁₀ h/μm≦log₁₀(N_(c) /mm ²)  (1)

0.01μm≦h≦1μm

A _(h1)=0.05; B _(h1)=3.3

[0029] and

[0030] c) the outer layer C has an acrylate-containing layer D.

[0031] The inventive film is preferably transparent and has an A-B-C-Dlayer structure. It is produced by coextrusion.

[0032] The layer A preferably contains no external particles and thefilm preferably has a planar orientation Δp of greater than 0.163.

[0033] In the context of the present invention, elevations are conicalelevations which protrude from the planar, oriented film surface.

[0034] By using the crosslinked, acrylate-containing coating D on atleast the surface layer C, both the roll formation and theprocessibility of the film can be distinctly improved, without worseningthe outstanding optical properties of the film. It has also been foundthat the outstanding optical properties of the film are not diminishedwhen the regrind which occurs (and may contain portions of the acryliccoating) in the course of film production is reused in the form of itsown regrind for the base layer B of the film.

[0035] According to the invention, the film is three- or four-layered.Its layers include a base layer B, a substantially pigment-free,high-gloss surface layer A and, on the other side of the base layer B, afurther layer C, preferably composed of polyethylene terephthalate. Thelayer C comprises the pigments required for the production andprocessing of the film.

[0036] It is possible in principle to use different raw materials forthe different layers. However, preference is given to producing theindividual layers on the basis of polyester raw materials.

[0037] The base layer B of the film consists of at least 80% by weightof a thermoplastic polyester. Suitable for this purpose are polyestersof ethylene glycol and terephthalic acid (polyethylene terephthalate,PET), of ethylene glycol and naphthalene-2,6-dicarboxylic acid(polyethylene 2,6-naphthalate, PEN), of 1,4-bishydroxymethylcyclohexaneand terephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate,PCDT) and also of ethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters whichconsist of at least 90 mol %, preferably at least 95 mol %, of ethyleneglycol and terephthalic acid units or of ethylene glycol andnaphthalene2,6-dicarboxylic acid units. The remaining monomer units stemfrom other aliphatic, cycloaliphatic or aromatic diols, or otherdicarboxylic acids, as may also occur in layers A or C.

[0038] The base layer preferably consists of PET.

[0039] Suitable other aliphatic diols are, for example, diethyleneglycol, triethylene glycol, aliphatic glycols of the general formulaHO—(CH₂)_(n)—OH where n is an integer from 3 to 6 (in particularpropane-1,3-diol, butane-1,4-diol, pentane-1,5-diol and hexane-1,6-diol)or branched aliphatic glycols having up to 6 carbon atoms. Of thecycloaliphatic diols, mention should be made of cyclohexanediols (inparticular cyclohexane-1,4-diol). Suitable other aromatic diolscorrespond, for example, to the formula HO—C₆H₄—X—C₆H₄—OH, where X is—CH₂—, —C(CH₃)₂—C(CF₃)₂—O—, —S— or —SO₂—. Also suitable in addition arebisphenols of the formula HO—C₆H₄—C₆H₄—OH.

[0040] Other aromatic dicarboxylic acids are preferablybenzenedicarboxylic acids, naphthalenedicarboxylic acids (for examplenaphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylicacids (in particular biphenyl-4,4′-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylicacids. Of the cycloaliphatic dicarboxylic acids, mention should be madeof cyclohexanedicarboxylic acids (in particularcyclohexane-1,4-dicarboxylic acid). Of the aliphatic dicarboxylic acids,the (C₃-C₁₉)-alkanedioic acids are particularly suitable, and the alkanemoiety may be straight-chain or branched.

[0041] The polyesters may be prepared by the known transesterificationprocesses (see Polyesters, V. V. Korshak and S. V. Vinogradova, PergamonPress, 1965, and Encyclopedia of Chemical Technology, Fourth Edition,Vol. 19, Pigments to Powders, Handling, Wiley & Sons). In this process,the starting materials are dicarboxylic esters and diols which arereacted with the customary transesterification catalysts such as saltsof zinc, calcium, lithium, magnesium and manganese. The intermediatesare then polycondensed in the presence of generally customarypolycondensation catalysts such as antimony trioxide or titanium salts.The preparation may equally be by the direct esterification process inthe presence of polycondensation catalysts. This starts directly fromthe dicarboxylic acids and the diols.

[0042] Particularly suitable processes have been found to be those inwhich transesterification catalysts are used which only slightly disruptthe smoothness of the surface A of the film. Preference is given inparticular to magnesium and manganese salts. These transesterificationcatalysts are preferably used in the preparation of the basis rawmaterial, but more preferably in the preparation of the raw material forthe layer A.

[0043] Films having the required smooth surface A are obtained inparticular when raw materials/polymers are used which have beenprepared, for example, using Mn, Mg or Ca transesterification catalystsand in which the Mn content is in the range from 50 to 200 ppm or the Mgcontent is in the range from 100 to 300 ppm or the Ca content is in therange from 50 to 300 ppm. This generates internal particles which leadsto films having the inventive surface (R_(a)≦28 nm, preferably ≦25 nm,more preferably ≦21 nm). However, it is also possible in principle touse other transesterification catalysts.

[0044] It is possible in principle to use the same polymers for layer Aas for the base layer B. Other materials may also additionally bepresent in the layer A, in which case the layer A then preferablyconsists of a mixture of polymers, a copolymer or a homopolymer whichpreferably contain ethylene 2,6-naphthalate units and/or ethyleneterephthalate units. Up to 10 mol % of the polymers may consist offurther comonomers, as have been described for the base layer B.

[0045] It is possible in principle to use the same polymers for theother layer (outer layer C) as have been described previously for thebase layer B or the layer A. For the processing of the polymers, it hasbeen found to be advantageous to select the polymers for the base layerand the two other layers in such a way that the viscosities of theparticular polymer melts do not differ too greatly. Otherwise,additional unevenness, disruption to flow or streak formation on thefinished film are to be expected in some circumstances. To describe theviscosity ranges of the melts for the particular layers, the intrinsicviscosity IV is used. For commercial polyethylene terephthalates whichare suitable with preference for producing the biaxially oriented films,the IV values are in the range from 0.54 to 0.76. In order to ensureimpeccable quality of the film for the purposes of the presentinvention, the SV value of the polymers for the layers A or C should bein the range from approximately 0.55 to 0.75, preferably in the rangefrom 0.56 to 0.74, especially preferably in the range from 0.57 to 0.73.If necessary, a solid phase condensation can be carried out on theindividual granules, in order to attain the required IV values of thematerials. The IV values of the polymer melts for the base layer and thetwo outer layers should differ preferably by not more than 0.1, morepreferably by not more than 0.08, but in particular by not more than0.06, IV unit.

[0046] The base layer B and the two other layers may additionallycontain customary additives, for example stabilizers. They areadvantageously added to the polymer or to the polymer mixture beforethey are melted. The stabilizers used are, for example, phosphoruscompounds such as phosphoric acid or phosphoric esters. The surfacelayer A of the film of the present invention preferably contains noantiblocking agents or other external particles. The surface layer C ofthe film of the present invention contains antiblocking agents in acertain concentration and in a certain size. The base layer containssubstantially only antiblocking agents which are introduced into thebase layer via regeneration of the film, i.e. when film offcut (regrind)is used, for example, to produce the film.

[0047] Typical antiblocking agents (also referred to in this context aspigments) are inorganic and/or organic particles, for example calciumcarbonate, amorphous silica, talc, magnesium carbonate, bariumcarbonate, calcium sulfate, barium sulfate, lithium phosphate, calciumphosphate, magnesium phosphate, aluminum oxide, LiF, calcium, barium,zinc or manganese salts of the dicarboxylic acids used, carbon black,titanium dioxide, kaolin or crosslinked polystyrene or acrylateparticles.

[0048] It is also possible to select mixtures of two and more differentantiblocking agents or mixtures of antiblocking agents of the samecomposition, but different particle size. The particles can be added tothe individual layers in the concentrations which are advantageous ineach case, for example as a glycolic dispersion, during thepolycondensation or via masterbatches in the course of extrusion. Adetailed description of the antiblocking agents which can be used can befound, for example, in EP-A-0 602 964.

[0049] Preferred particles for the outer layer C are SiO₂ in colloidaland in chainlike form. The particle diameters of the particles used arein principle not restricted. For the achievement of the object, it hasbeen found to be advantageous to use

[0050] particles having an average primary particle diameter of lessthan 60 nm, preferably less than 55 nm and more preferably less than 50nm and/or

[0051] particles having an average primary particle diameter (the d₅₀value) in the range from 1 to 4 μm, preferably in the range from 1.5 to3.5 μm and more preferably in the range from 2.5 to 3.0 μm.

[0052] It has also been found to be particularly advantageous when thedistribution of the particle diameter lies within certain limits. Toachieve very low opacity and very high gloss, the outer layer Ccomprises a pigment system for which the variance in diameter d(expressed by the SPAN 98) is preferably less than 1.9.

[0053] The pigment concentration in the outer layer C is preferablybetween 0.1 and 0.5% by weight, advantageously between 0.15 and 0.45% byweight, in particular between 0.2 and 0.4% by weight and most preferablybetween 0.25 and 0.35% by weight, based on the weight of layer C. Itdepends in particular on the desired processing performance of the film.Preference is given to selecting the pigment type(s), the pigmentconcentration(s) and the particle concentration(s), and also the layerthickness ratios, in such a way that the film has good opticalproperties and also good producibility and processibility. Theseparameters can be determined simply and reliably by a few preliminaryexperiments.

[0054] The pigments for the surface layer C are to be selected withrespect to size, concentration and their distribution in such a way thatthe number of elevations N_(c)/mm² can be described by the followingequations (h=height of the elevations):

A _(h1)−B_(h1)·log₁₀ h/μm≦log₁₀(N_(c) /mm ²)  (1)

0.01μm≦h≦1μm

[0055] Constants A_(h1), B_(h1) A_(h1) B_(h1) inventive 0.05 3.3preferred 0.624 3.229 more preferred 1.1 3 most preferred 2.477 2.22

[0056] In an advantageous embodiment of the invention, the number ofelevations N_(c) /mm ² can be described by the following equation (2):

A _(h1) −B _(h1)·log₁₀ h/μm≦log ₁₀(N _(c) /mm ²)≦A _(h2) −B _(h2)·log ₁₀h/μm  (2)

0.01μm≦h≦1μm

[0057] Constants A_(h1), B_(h1), Ah_(h2), Bh_(h2) A_(h1) B_(h1) A_(h2)B_(h2) inventive 0.05 3.3 4.08 1.5 preferred 0.624 3.229 4.08 1.5 morepreferred 1.1 3 4.08 1.5 most preferred 2.477 2.22 4.08 1.5

[0058] When the outer layer C of the film of the present invention has atopography in which the number of elevations is below the rangeindicated by the equation (1), the film is difficult to produce and theprocessing performance is often inadequate. The film tends to block.

[0059] In the film of the present invention, the thicknesses of theouter layers A and C are generally greater than 0.6 μm and arepreferably in the range from 0.6 to 2.5 μm, advantageously in the rangefrom 0.7 to 2.3 μm, in particular in the range from 0.8 to 2.2 μm andmost preferably in the range from 0.9 to 2.1 μm. The outer layers A andC may have the same or different thickness.

[0060] The total thickness of the polyester film according to theinvention may vary within wide limits and depends on the intendedapplication. It is preferably from 4 to 50 μm, in particular from 5 to45 μm, with preference from 6 to 40 μm, and the layer B has a proportionof preferably from 5 to 95% of the total thickness.

[0061] According to the invention, at least one side of the film,preferably the rough surface C, has been coated with a preferablyaqueous dispersion. The coating D on the finished film has a thicknessof from approx. 5 to 2000 nm, preferably from 10 to 500 nm, inparticular from 20 to 200 nm. The coating is preferably applied inline,i.e. during the film production process, appropriately before thetransverse stretching. Particular preference is given to applying thecoating by means of the reverse gravure-roll coating method in which thecoatings can be applied very homogeneously in layer thicknesses of up to100 nm. Preference is likewise given to applying the coating by theMeyer rod method (literature on the coating methods: Die Kunststoffe,Kunststoffhandbuch, Dr. Bodo Carlowitz, Hanser, 1990), by whichrelatively large coating thicknesses can be achieved. The coating isapplied preferably as a solution, suspension or dispersion, morepreferably as an aqueous solution, suspension or dispersion. The coatingmentioned confers on the film surface or on the film the desiredfunctions (low coefficient of friction, good processibility, good rollformation, low static charge) and possible further functions. Forexample, the printability of the film can be improved or it can beprovided with an improved aroma barrier or adhesion can be made possibleto materials which would not otherwise adhere to the uncoated filmsurface (for example photographic emulsions).

[0062] The materials/compositions mentioned are applied to one or bothfilm surfaces as a dilute solution, emulsion or dispersion, preferablyas an aqueous solution, emulsion or dispersion, and the solvent/thedispersant is subsequently volatilized. When the coating is applied inline before the transverse stretching, the heat treatment in thetransverse stretching and subsequent heat setting is commonly sufficientto volatilize the solvent/the dispersant and dry the coating.

[0063] The dispersion comprises acrylic copolymers as a solidconstituent. The copolymers used with preference consist preferablysubstantially of at least 50% by weight of one or more polymerizedacrylic and/or methacrylic monomers and of from approx. 1 to 15% byweight of copolymerizable comonomers which, in the copolymerized state,are capable of developing intermolecular crosslinking under the actionof elevated temperature, in some cases without addition of a specialresinous crosslinking agent. The acrylic copolymers may in principlealso contain further monomer units.

[0064] The acrylic component of the copolymers is present preferably inan amount of from 50 to 99% by weight and preferably consists of anester of methacrylic acid, in particular an alkyl ester, whose alkylgroup contains up to 10 carbon atoms, for example the methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, tertiary butyl, hexyl,2-ethylhexyl, heptyl and n-octyl group. Acrylic copolymers which arederived from a lower alkyl acrylate (C₁ to C₄), in particular ethylacrylate, together with a lower alkyl methacrylate, provide particularlygood adhesion between the polyester film and reprographic coatings andmatt coatings applied thereto. Very particular preference is given tousing adhesion promoter copolymers of an alkyl acrylate, e.g. ethylacrylate or butyl acrylate, together with an alkyl methacrylate, e.g.methyl methacrylate, in particular in the same molar proportions and ina total amount of from approx. 70 to 95% by weight. The acrylatecomonomer of such acrylic/methacrylic combinations is present preferablyin a proportion of from 15 to 65 mol % and the methacrylate comonomerpreferably in a proportion which is generally from 5 to 20 mol % greaterthan the proportion of the acrylate comonomers. The methacrylate ispreferably present in the combination in a proportion of from 35 to 85mol %.

[0065] Comonomers which are suitable in accordance with the inventionand are also used for increasing the solvent resistance andintermolecular crosslinking of the coating are, for example,N-methylolacrylamide, N-methylolmethacrylamide and the correspondingethers; epoxide materials, e.g. glycidyl acrylate, glycidyl methacrylateand allyl glycidyl ether; monomers containing carboxyl groups, e.g.crotonic acid, itaconic acid or acrylic acid; anhydrides, e.g. maleicanhydride; monomers containing hydroxyl groups, e.g. itaconic anhydride;monomers containing hydroxyl groups, e.g. allyl alcohol and hydroxyethylor hydroxypropyl acrylate or methacrylate; amides, e.g. acrylamide,methacrylamide or maleamide and isocyanates, e.g. vinyl isocyanate orallyl isocyanate. Of the abovementioned crosslinking comonomers,preference is given to N-methylolacrylamide andN-methylolmethacrylamide, primarily because copolymer chains whichcontain one of these monomers are able under the action of elevatedtemperatures to condense together and thus to develop the desiredintermolecular crosslinking. However, the solvent resistance which isdesired in some cases of the preferred acrylate coating can also beachieved by the presence of an extraneous crosslinking agent, forexample a melamine- or urea-formaidehyde condensation product. When nosolvent resistance is required, crosslinking agents can be dispensedwith.

[0066] The above-described inventive coating is illustrated in detail inU.S. Pat. No. 4,571,363, United States equivalent of EP-A-0 144 948,which is explicitly incorporated herein at this point (cf. in particularpage 5 to page 17). This document also gives information about furtherspecific combinations of such mixed acrylic copolymers, which are notreproduced here but are explicitly encompassed by the invention. Thiscoating is applied in accordance with the object set preferably to thesurface of the outer layer C (the side which is not coated with barriersubstances) of the film. However, it is also possible to also providethe other side (the outer layer A) with the inventive coating or toapply another coating to this side. The coating formulation may containknown additives, for example antistats, wetting agents, surfactants, pHregulators, antioxidants, dyes, pigments, antiblocking agents, forexample colloidal SiO₂, etc. It is normally appropriate to incorporate asurfactant, in order to increase the ability of the aqueous coating towet the polyester carrier film.

[0067] In the film of the present invention, the coefficient of friction(COF) of the film side C which bears the acrylic coating is particularlylow. On this (coated side), the coefficient of friction is generallyless than 0.45, preferably less than 0.42 and more preferably less than0.4. The low coefficient of friction makes an important contribution togood processing performance of the film and to good roll formation.

[0068] A particular advantage is that the production costs of the filmaccording to the invention are only slightly above those of a film madeof standard polyester raw materials. The properties of the filmaccording to the invention which are relevant to processing and use aredistinctly improved compared to prior art films.

[0069] In addition, when producing the film, it is guaranteed that theregrind can be reused in a proportion of up to approx. 60% by weight,preferably from 10 to 50% by weight, based in each case on the totalweight of the film, without significantly adversely affecting thephysical properties of the film.

[0070] The present invention therefore also provides a process forproducing the films according to the invention. It comprises

[0071] producing a multilayer film from a base layer B and the outerlayer(s) A (and C) by coextrusion and shaping the melts to give flatmelt films,

[0072] coating the film with the layer which crosslinks via acryliccompounds (preferably between the first and the second stretching step),

[0073] biaxial stretching of the film, and

[0074] heat-setting of the stretched film.

[0075] To produce the layers A and C (outer layers A and C), it isappropriate to feed polymer granules, preferably of polyethyleneterephthalate, to two extruders. The materials are melted at about 300°C. and extruded or coextruded.

[0076] The polymers for the base layer are appropriately fed through afurther extruder. Any foreign bodies or impurities present can befiltered out of the polymer melt. The melts are shaped in a multilayernozzle to give flat melt films and layered on top of each other.Subsequently, the multilayer film is drawn off with the aid of a chillroll and optionally further rolls and solidified.

[0077] The biaxial stretching is generally carried out sequentially.Preference is given to stretching first in the longitudinal direction(i.e. in machine direction, MD) and then in transverse direction (i.e.at right angles to machine direction, TD). The stretching in thelongitudinal direction can be carried out with the aid of two rollsrunning at different rates in accordance with the intended stretchingratio. For transverse stretching, an appropriate center frame isgenerally used.

[0078] The temperature at which the stretching is carried out can bevaried within a relatively wide range and depends on the desiredproperties of the film. In general, the longitudinal stretching iscarried out at from approx. 80 to 130° C. and the transverse stretchingat from approx. 80 to 150° C. The longitudinal stretching ratio ispreferably in the range from 3:1 to 6:1, in particular from 3.5:1 to5.5:1. The transverse stretching ratio is generally in the range from3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1.

[0079] The film of the present invention is preferably oriented in sucha way that the planar orientation Δp of the film is greater than 0.163,preferably greater than 0.164 and more preferably greater than 0.165.Otherwise, the gloss of the surface layer A is in some circumstancesinadequate for barrier coating by the CVD method.

[0080] In the subsequent heat-setting, the film is kept at a temperatureof from approx. 150 to 250° C. for from about 0.1 to 10 s. Subsequently,the film is wound up in a customary manner.

[0081] One advantage of the invention is that the production costs ofthe inventive film are comparable with those of the prior art. The otherproperties of the inventive film which are relevant to processing anduse remain substantially unchanged or are even improved. In addition,when producing the film, it is guaranteed that the regrinds can bereused in a concentration of from approx. 20 to 50% by weight, based onthe total weight of the film, without the physical properties of thefilm being significantly adversely affected.

[0082] The film is outstandingly suitable, for example, for thepackaging of foods or other consumable items which are sensitive to airand/or to light. It is especially suitable for producing packagings forcoffee, tea or spices, in particular ground coffee.

[0083] In summary, the inventive film features an outstanding oxygenbarrier after it has been coated on film surface A with materials whichare used in CVD or in PECVD methods, a high gloss of the film surface Aand a low opacity. In addition, the film which has been coatedbeforehand with the above-described acrylic polymers has very goodhandling, very good winding properties and very good processingperformance. The film also becomes distinctly less electrostaticallycharged than comparable films which do not have the above-describedlayer which crosslinks via acrylic compounds. The layer which crosslinksvia acrylic compounds is additionally especially suitable for printing,in particular with nitrocellulose(NC) dyes.

[0084] The film has an oxygen permeation of less than 3.0cm³·m⁻²·d⁻¹·bar⁻¹, preferably less than 2.5 cm³·m⁻²·d⁻¹·bar⁻¹ and morepreferably less than 2.0 cm³·m⁻²·d⁻¹·bar⁻¹ after it has been coated onthe film surface A; preferably by the CVD or PECVD method.

[0085] Substances which can be applied to the surface of the outer layerby the CVD or PECVD method are, for example, hexamethyldisiloxane orCH₄, and their appropriate method is the AIRCO Coating Technology whichwas presented, for example, by R. J. Nelson at the Society for theResearch of Polymerized Compound Surfaces, Fukui City, Japan, Jul. 9,1993.

[0086] The gloss (20°) of the film surface A is preferably greater than190. In a preferred embodiment, the gloss of this side is more than 200and in a particularly preferred embodiment more than 210. This filmsurface is also suitable for printing or for metallization. The highgloss of the film is transferred to the print or the applied metal layerand thus confers on the film the desired appearance which is effectivein advertising. The opacity of the film is preferably less than 2.5%.

[0087] The table which follows (table 1) once again summarizes the mostimportant inventive film properties. TABLE 1 Inventive Most rangePreferred More preferred preferred Unit Measurement method Constants ofthe equation A_(h1) = 0.05 A_(h1) = 0.624 A_(h1) = 1.1 A_(h1) = 2.477(1) for the side C, which is B_(h1) = 3.3 B_(h1) = 3.229 B_(h1) = 3.0B_(h1) = 2.22 not to be coated Oxygen permeation of the <3 <2.5 <2 cm³ ·m⁻² · d⁻¹ · bar⁻¹ DIN 53 380, part 3 coated film Gloss, sideA >190 >200 >210 DIN 67 530 (measuring angle 20°)^(i)) Opacity^(i)) <2.5<2.0 <1.7 % ASTM-D 1003-52 Coefficient of friction: <0.45 <0.42 <0.39DIN 53 375 C side against itself Coefficient of friction: <0.6 <0.55<0.5 DIN 53 375 A side against itself Average roughness R_(a) 30-15040-123 50-110 nm DIN 4768 at a cutoff of C side 0.25 nm Averageroughness R_(a) 28 25 21 nm DIN 4768 at a cutoff of A side (i) 0.25 nm

[0088] To characterize the raw materials (starting polymers) and thefilms, the following methods were used:

[0089] DIN=Deutsches Institut für Normung [German Institute forStandardization]

[0090] ASTM=American Society for Testing and Materials

[0091] ISO=International Organization for Standardization

[0092] (1) Determination of the Planar Orientation Δp

[0093] The planar orientation is determined via the measurement of therefractive index with an Abbe refractometer (A. Krüss, Optronic,Hamburg, Germany).

[0094] The sample preparation is as follows:

[0095] Sample size and sample length: 60 to 100 nm

[0096] Sample breadth: corresponds to prism breadth of 10 mm

[0097] To determine n_(MD) and n_(α)(=n_(z)), the sample to be analyzedhas to be cut out of the film with the running edge of the samplecoinciding exactly with TD. To determine n_(TD) and n_(α), (=n_(z)), thesample to be analyzed has to be cut out of the film with the runningedge of the sample coinciding exactly with MD. The samples are to betaken from the middle of the film web. Care has to be taken that theAbbe refractometer has a temperature of 23° C. With the aid of a glassrod, a little diiodomethane (n=1.745) or diiodomethanebromonaphthalenemixture is applied before the measurement to the thoroughly cleanedlower prism. The refractive index of the mixture has to be greater than1.685. First the sample cut out in TD is applied thereto, in such a waythat the entire prism surface is covered. A paper tissue is then used tofasten the film firmly onto the prism, so that the film lies firm andsmooth. The excess liquid has to be sucked off. Afterwards, a little ofthe measuring liquid is dripped onto the film. The second prism is swungdownwards and pressed on firmly. The right-hand knurled screw is thenused to turn the indicator scale until a transition from light to darkcan be seen in the viewing window in the range from 1.62 to 1.68. Whenthe transition from light to dark is not sharp, the colors are broughttogether with the aid of the upper knurled screw in such a way that onlya light and a dark zone are visible. The sharp transition line isbrought with the aid of the lower knurled screw to the crossing point ofthe two diagonal lines (in the eyepiece). The value now indicated in themeasuring scale is read off and entered into the test record. This isthe refractive index in machine direction n_(MD). The scale is nowturned with the lower knurled screw until the range visible in theeyepiece is between 1.49 and 1.50.

[0098] The refractive index n_(α)or n_(z), (in the thickness directionof the film) is now determined. So that the transition, which is onlyweakly visible, can be better seen, a polarization film is placed on theeyepiece. This has to be turned until the transition can be clearlyseen. The same applies as for the determination of n_(MD). When thetransition from light to dark is not sharp (colored), the colors arebrought together with the aid of the upper knurled screw in such a waythat a sharp transition can be seen. This sharp transition line isbrought with the aid of the lower knurled screw to the crossing point ofthe two diagonal lines and the value indicated on the scale is read offand entered into the table.

[0099] Subsequently, the sample is turned and the correspondingrefractive indices n_(MD) and n_(α)(=n_(z)) of the other surface sideare measured and entered into a corresponding table.

[0100] After the determination of the refractive indices in MD and inthe thickness direction, the sample strip cut out in MD is laid on andthe refractive indices n_(TD) (at right angles to machine direction) andn_(α)(=n_(z)) are determined correspondingly. The strip is turned roundand the values are measured for the B side. The values for the A sideand the B side are combined to average refractive values. Theorientation values are then calculated from the refractive indices bythe following formulae:

Δn=n _(MD) −n _(TD)

Δp=(n _(MD) +n _(TD))/2−n _(z)

n _(av)=(n _(MD) +n _(TD) +n _(z))/3

[0101] (2) Oxygen Permeation

[0102] The oxygen permeation of the coated films was measured using anOX-TRAN 2/20 from Mocon Modern Controls (USA) in accordance with DIN 53380, part 3.

[0103] (3) SV (Standard Viscosity) Value and IV Value

[0104] The standard viscosity SV (DCA) is determined at 25° C. indichloroacetic acid, based on DIN 53726. The intrinsic viscosity (IV) iscalculated from the standard viscosity as follows:

IV=[η]=6.907·10⁻⁴ SV(DCA)+0.063096[dl/g]

[0105] (4) Friction

[0106] The friction was determined to DIN 53375. The friction wasdetermined 14 days after the production. Blocking takes place when thefrictional value is greater than 1 or when discontinuities occur whenfrictional force is measured in the variation of frictional force withdisplacement.

[0107] (5) Opacity

[0108] The opacity of the film was determined to ASTM-D 1003-52. Theopacity measurement according to Hölz was determined based on ASTM-D1003-52, except that, to utilize the optimum measuring range,measurement was effected on four film plies lying on top of one anotherand, instead of a 4° pinhole diaphragm, a 1° slot diaphragm is used.

[0109] (6) Gloss

[0110] The gloss was determined to DIN 67530. The reflector value wasmeasured as a characteristic optical parameter for the surface of afilm. Based on the standards ASTM-D 523-78 and ISO 2813, the angle ofincidence was set to 20° or 60°. A light beam hits the flat test surfaceat the angle of incidence set and is reflected or scattered by it. Thelight beams incident upon the photoelectronic detector are displayed asa proportional electrical parameter. The measurement is dimensionlessand has to be quoted together with the angle of incidence.

[0111] (7) Determination of the Particle Sizes on Film Surfaces

[0112] The size distribution of elevations on film surfaces isdetermined using a scanning electron microscope and an image analysissystem. The system used is the Philips XL30 CP scanning electronmicroscope with an integrated image analysis program AnalySIS fromSoft-imaging System.

[0113] For these measurements, film samples were applied flat to asample holder. These are subsequently provided with a thin metal layer(for example of silver) by vapor deposition obliquely at an angle α. αis the angle between sample surface and the diffusion direction of themetal vapor. This oblique vapor deposition results in a shadow behindthe elevation. Since the shadows are not yet electrically conductive,the sample is subsequently subjected to vapor deposition or sputteringwith a second metal (e.g. gold), which results in the second coatinghitting the sample surface vertically and therefore no shadows beingpresent in the second coating.

[0114] The sample surfaces prepared in this way are imaged in a scanningelectron microscope (SEM). As a consequence of the material contrast ofthe metals, the shadows of the elevations are visible. The sample isoriented in the SEM in such a way that the shadows run parallel to oneedge of the image. For the image recording, the following SEM conditionsare set: secondary electron detector, operating distance: 10 mm,acceleration voltage: 10 kV and spot: 4.5. The brightness and contrastare adjusted in such a way that all image information is represented asgray values and the intensity of the background noise is so small thatit is not detected as shadow. The length of the shadows is evaluatedusing the image analyzer. The threshold value for the shadow recognitionis placed at the point where the second derivative of the gray valuedistribution of the image passes through the zero point. Before theshadow recognition, the image is smoothed using an N×N filter (size 3, 1iteration). The setting of a frame ensures that elevations that are notdepicted fully in the image are not included in the measurement. Themagnification, the frame size and the number of images evaluated areselected in such a way that a total of 0.36 mm² of film surface isevaluated. The height of the individual elevations is calculated fromthe individual shadow lengths by the following relationship:

h=tan(α)·L

[0115] where h is the height of the elevation, α is the vapor depositionangle and L is the shadow length. The elevations determined in this wayare divided into classes, in order to arrive at a frequencydistribution. The division is into classes of breadth 0.05 μm between 0and 1 μm, and the smallest class (0 to 0.05 μm) is not used for furtherevaluations. The diameters (diffusion at right angles to the directionin which the shadow is thrown) of the elevations are classified in asimilar manner into classes of breadth 0.2 μm from 0 to 10 μm, and inthis case also the smallest class is not used for further evaluation.

[0116] (8) Roughness

[0117] The roughness R_(a) of the film was determined to DIN 4768 at acutoff of 0.25 mm.

[0118] (9) Measurement of the Average Particle Size d₅₀

[0119] The determination of the average particle size d₅₀ was carriedout by means of laser on a Malvern MasterSizer (Malvern InstrumentsGmbH, Herrenberg, Germany) by the standard method (other measuringinstruments are, for example, Horiba LA 500 or Sympathec Helos, whichuse the same measuring principle). To this end, the samples wereintroduced into a cuvette with water and this was then placed in themeasuring instrument. The measuring procedure is automatic and includesa mathematical determination of the d₅₀ value.

[0120] The d₅₀ value is determined by definition from the (relative)cumulative particle size distribution curve: the point at which the 50%ordinate value cuts the cumulative curve immediately provides thedesired d₅₀ value on the abscissa axis.

[0121] (10) Measurement of the SPAN 98

[0122] The determination of the SPAN 98 was carried out with the samemeasuring instrument as above for the determination of the averagediameter d₅₀. The SPAN 98 is defined as follows:${SPAN98} = {\frac{d_{98} - d_{10}}{d_{50}}.}$

[0123] For the determination of d₉₈ and d₁₀, the basis is again the(relative) particle size distribution curve. The point at which the 98%ordinate value cuts the cumulative curve immediately provides thedesired d₉₈ value on the abscissa axis and the point at which the 10%ordinate value cuts the cumulative curve immediately provides thedesired d₁₀ value on the abscissa axis.

EXAMPLE 1

[0124] Chips of polyethylene terephthalate (prepared by thetransesterification process using Mn as the transesterificationcatalyst, Mn concentration: 100 ppm) were dried at 160° C. to a residualmoisture of below 50 ppm and fed to the extruder for the base layer B.

[0125] In addition, chips of polyethylene terephthalate (prepared viathe transesterification process using Mn as the transesterificationcatalyst, Mn concentration: 100 ppm) were likewise dried at 160° C. to aresidual moisture of below 50 ppm and fed to the particular extrudersfor the outer layers A and C. The granules for the outer layer A containno external pigments; the granules for the outer layer C contain thepigments required for the processing of the film.

[0126] Coextrusion and subsequent stepwise orientation in thelongitudinal and transverse direction provided a transparent three-layerfilm having ABC structure and a total thickness of 12 μm. The thicknessof the particular layers can be taken from table 2.

[0127] The inventive coating D consists of a 4.5% by weight solution ofa latex consisting of a copolymer of 60% by weight of methylmethacrylate, 35% by weight of ethyl acrylate and 5% by weight ofN-methylolacrylamide. The outer layer C of the film was coated inaccordance with the present invention between the longitudinal andtransverse orientation of the film, using the reverse gravure-rollcoating method. The layer thickness of the coating D on the final filmwas approx. 50 nm.

[0128] Outer layer A, mixture of: 100.0% by weight of polyethyleneterephthalate having an IV value of 0.616

[0129] Base layer B: 100.0% by weight of polyethylene terephthalatehaving an IV value of 0.616

[0130] Outer layer C, mixture of: 88.0% by weight of polyethyleneterephthalate having an IV value of 0.616 12.0% by weight of amasterbatch composed of 98.0% by weight of polyethylene terephthalateand 1.0% by weight of Sylobloc® 44 H (Grace) and 1.0% by weight ofAerosil® TT 600 (Degussa); these are both SiO₂.

[0131] The production conditions in the individual process steps were:Extrusion: Temperatures A layer: 300° C. B layer: 300° C. C layer: 300°C. Die gap width: 1 mm Temperature of the takeoff roll: 30° C.Longitudinal stretching: Temperature: 80-115° C. (heating 80-115° C.,stretching at 115° C.) Longitudinal stretching ratio: 4.5 Transversestretching: Temperature: 80-155° C. Transverse stretching ratio: 4.0Setting: Temperature: 230° C. Time: 3 s

[0132] After producing the film (of this example 1 and the followingexamples), it was coated on the A side in an industrial PE-CVD coaterwith SiO_(x) by means of the plasma polymerization ofhexamethyldisiloxane under reduced pressure. The coating rate was 5 m/s.

[0133] The film had the required high oxygen barrier (=low oxygenpermeation). The film structure and the properties achieved of the filmsproduced can be taken from tables 2 and 3.

EXAMPLE 2

[0134] In a similar manner to example 1, coextrusion and subsequentstepwise orientation in the longitudinal and transverse direction wereused to produce a transparent, three-layer film having ABC structure anda total thickness of 12 μm. In comparison to example 1, only outer layerC was changed.

[0135] Outer layer C, mixture of:

[0136] 80.0% by weight of polyethylene terephthalate having an SV valueof 800

[0137]20.0% by weight of a masterbatch of 98.0% by weight ofpolyethylene terephthalate (SV value of 800) and 1.0% by weight ofSylobloc®44 H (Grace) and 1.0% by weight of Aerosil® TT 600 (Degussa)

[0138] The process conditions for the film for all layers were asselected in example 1.

Comparative Example 1 (CE 1)

[0139] Example 1 of EP-A-0 514 129 was reproduced. The film which hadbeen coated as in example 1 and was 75 pm thick did not have therequired oxygen permeation. The gloss of the film is not satisfactory.

Comparative Example 2 (CE 2)

[0140] Example 1 of EP-A-0 604 057 was reproduced. The film which hadbeen coated as in example 1 and was 60 μm thick did not have therequired oxygen permeation. The gloss of the film is not satisfactory.TABLE 2 Layer Average pigment Pigments Film thickness diameterconcentration thickness Film A B C Pigments in the layers A B C A B CExample μm structure μm A B C μm ppm Example 1 12 ABC 1.0/9.5/1.5 nonenone Sylobloc 44H 2.5 0 0 1e+07 Aerosil TT 600 0.04 Example 2 12 ABC1.0/9.5/1.5 none none Sylobloc 44H 2.5 0 0 2e+07 Aerosil TT 600 0.04 CE175 ABA 4/67/4 Glass beads + none Glass beads + 2.7 2.7 3001 0 3e+06Aerosil Ox50 Aerosil Ox50 0.04 0.04 200 CE2 60 ABA 1.5/57/1.5 Tosperl130 none Tosperl 130 3.0 3.0 600 0 600

[0141] TABLE 3 Constants for the height distribution Roughness Roughnessof the particles Oxygen Planar R_(a) R_(a)(i) Gloss¹⁾ A_(h) permeationFriction orientation C side A side (20°) Opacity¹⁾ Processing Examples Cside B_(h) cm³/(m² · bar · d) C/C Δp nm nm A side % performance Example1 A_(h1) = 0.65 B_(h1) = 3.0 0.5* 0.35* 0.166 60 20 200 1.7 very goodExample 2 A_(h1) = 1.2 B_(h1) = 2.9 0.5* 0.32* 0.167  65* 20 200 1.9*very good CE1 3.5 0.46 35 160 1.3 CE2 4 0.26 55 165 0.6

1. A biaxially oriented polyester film which has a base layer B composedof at least 80% by weight of a thermoplastic polyester, and has twoouter layers A and C, wherein a) the outer layer A has high surfacesmoothness and comprises substantially no external particles, b) theouter layer C comprises external particles and has, per mm² of filmsurface area, a number of elevations N_(c) which correlate by way of thefollowing equation to their respective heights h A _(h1) −B_(h1)·log₁₀h/μm≦log₁₀(N _(c)/mm²)  (1)0.01μm ≦h≦1 μmA _(h1)=0.05; B_(h1)=3.3 and c) the outer layer C has an acrylate-containing layer D.2. The polyester film as claimed in claim 1, wherein the polyester ofthe base layer B contains units of ethylene glycol and terephthalicacid, and/or units of ethylene glycol and naphthalene-2,6-dicarboxylicacid.
 3. The polyester film as claimed in claim 1, wherein the polyesterused in the base layer B comprises polyethylene terephthalate.
 4. Thepolyester film as claimed in claim 1, wherein no external particles arepresent in the outer layer A.
 5. The polyester film as claimed in claim1, wherein the outer layer C comprises, as external particles,antiblocking agents or pigments.
 6. The polyester film as claimed inclaim 1, wherein the antiblocking agents present in the outer layer Ccomprise SiO₂.
 7. The polyester film as claimed in claim 1, wherein theexternal particles of the outer layer C have an average primary particlediameter smaller than 60 nm and/or an average primary particle diameterof from 1 to 4 μm.
 8. The polyester film as claimed in claim 1, whereinthe concentration of the external particles in the outer layer C is from0.1 to 0.5% by weight, based on the weight of the layer C.
 9. Thepolyester film as claimed in claim 1, wherein the acrylate-containinglayer D comprises one or more polymerized acrylic and/or methacrylicmonomers and copolymerizable comonomers capable of developingintermolecular crosslinking.
 10. The polyester film as claimed in claim1, wherein the acrylate-containing layer D is applied in the form of anaqueous dispersion to the outer layer C.
 11. The polyester film asclaimed in claim 1, which has a planar orientation greater than 0.163.12. The polyester film as claimed in claim 1, wherein the uncoatedsurface layer A has a gloss (20°) greater than 190 and a roughness Ra≦28nm.
 13. The polyester film as claimed in claim 1, which has an A-B-C-Dlayer structure.
 14. The polyester film as claimed in claim 1, whereinthe oxygen permeation provided by the film coated on the outer layer Aby the CVD or PECVD process is smaller than 3 cm³/(m²·bar·d).
 15. Thepolyester film as claimed in claim 1, wherein the outer layer A of thefilm has been coated, using plasma-polymerized hexamethyldisilazane orCH₄.
 16. A process for producing a polyester film as claimed in claim 1,comprising the steps of a) producing a multilayer film by extrusion orcoextrusion, and shaping the melts to give flat melt films, b) coatingthe film with an acrylate-containing layer, c) biaxial stretching of thefilm, and d) heat-setting of the stretched film.
 17. Packaging filmformed from polyester film in accordance with claim 1.